Among researches for improving interlayer performance of a composite material laminated structure or connection performance between at least two composite material laminated structures, a research into a Z-pinning technology has relatively recently started. Known technologies for the Z-pinning will be schematically described below.
A concept of a Z-pinning method has been first suggested in U.S. Pat. No. 4,808,461 (1989), a concept of inserting a pin by adding an ultrasonic vibration load has been suggested in U.S. Pat. No. 5,186,776 (1993), and a concept of manufacturing a Z-pinning composite material structure using an ultrasonic gun and a compressible foam into which a pin is inserted has been suggested in U.S. Pat. No. 5,589,015 (1996). Then, a Z-pinning concept has been developed in U.S. Pat. No. 5,667,859 (1997), U.S. Pat. No. 5,800,672 (1998), U.S. Pat. No. 5,919,413 (1999), U.S. Pat. No. 6,190,602 (2001), U.S. Pat. No. 6,291,049 (2001), U.S. Pat. No. 6,405,417 (2002), U.S. Pat. No. 6,436,507 (2002), and the like. However, in these patents, a compressible foam has been continuously used.
In Korea, a Z-pinning method that does not use the compressible foam described above has been suggested by the present applicant in Korea Patent No. 0932302 (Dec. 8, 2009) entitled “Composite Laminated Structure Reinforced by Inserting Pins, Method and Apparatus for Making the Same and Method for Making the Apparatus”, or the like.
In the Z-pinning according to the related art as described above, the Z-pin inserted into the composite material laminated structure is mainly made of a metal material or a composite material, and an appearance of the Z-pin has a shape in which a distal end portion is sharp and a pillar portion is smooth as shown in FIG. 1 showing a Z-pin 10 according to the related art. In the Z-pinning technology, it is very important to improve coupling force between the Z-pin and the composite material into which the Z-pin is inserted. Therefore, it is more preferable that the pillar portion of the Z-pin has an unevenness shape rather than a smooth shape. However, according to the related art, since sizes of the Z-pins themselves are small and a large number of Z-pins are required, it is not easy to form unevennesses on the Z-pin while appropriately using a cheap processing scheme.
Nevertheless, when the pillar portion of the Z-pin is smoothly formed, since coupling force between the Z-pin and the composite material can not but decrease, various researches for economically forming appropriately fine unevenness parts on the Z-pin have been conducted. As a part of these researches, a technology of forming the unevenness parts on the Z-pin using a method such as a chemical corrosion method, a method of mechanically shot-peening small steelies, or the like, has been disclosed.